This is a continuing study in the canine model of total body irradiation, marrow transplantation, methods of support for animals that have no bone marrow function and immunologic problems after marrow transplantation. The major areas of proposed research to be conducted under the renewed grant are as follows: I. Study the problem of resistance to hematopoietic grafts, conduct studies designed to overcome such resistance and improve the long-term success rate when donor-recepient pairs are not fully matched for the major histocompatibility complex. II. Study monoclonal antibody-radioactive isotope conjugates for their usefulness in conditioning dogs for marrow transplantation. III. Study high-dose total body irradiation and compare toxicities and immuno supperssive properties of fractionated versus single dose irradiation at different dose rates. The objective is to develop conditioning regimens for histoincompatible marrow transplant recipients and those with malignancies. IV. Prevention and treatment of graft-versus-host disease which is recognized as the principle obstacle to a wider application of marrow transplantation to the treatment of human diseases.